A flow volume loop is a graphical representation of a person’s breathing, plotting the speed of airflow against the volume of air in the lungs. This chart is generated during a lung assessment called spirometry. By analyzing the loop’s shape and measurements, doctors can identify potential issues with airflow or lung capacity.
The Spirometry Test Procedure
The process of generating a flow volume loop begins with a spirometry test, a straightforward and painless procedure. A patient sits upright while a technician places clips on their nose to ensure all breathing occurs through the mouth. They are then given a sterile mouthpiece connected to a spirometer.
A technician guides the patient through the required breathing maneuvers. This involves taking the deepest breath possible, sealing the lips tightly around the mouthpiece, and then blasting the air out as forcefully and quickly as possible. This is immediately followed by a rapid and full inhalation.
To ensure reliable results, the technician will have the patient repeat this maneuver several times. The spirometer records the data from each attempt, and the most consistent efforts are used for interpretation.
Anatomy of a Normal Flow Volume Loop
A normal flow volume loop has a distinct shape that provides a baseline for comparison. The graph has two axes: the vertical y-axis represents the rate of airflow in liters per second, and the horizontal x-axis shows the volume of air in liters. The loop has two main parts corresponding to exhalation and inhalation.
The top portion of the loop, representing exhalation, begins with a rapid climb to the highest point on the graph. This peak is the Peak Expiratory Flow (PEF), the fastest speed the person can exhale. Following the PEF, the curve descends in a relatively straight or slightly bowed-out line as the lungs empty.
The bottom portion of the loop illustrates forced inhalation. This curve is a symmetric, rounded, or saddle-shaped curve showing a more consistent flow rate than exhalation. The total width of the loop represents the Forced Vital Capacity (FVC), the maximum amount of air forcibly exhaled after a full inhalation.
Obstructive Lung Disease Patterns
In individuals with obstructive lung diseases such as asthma or COPD, the flow volume loop takes on a characteristic appearance. The most telling feature is a change in the expiratory part of the curve. Instead of a straight downward slope after the peak, the line becomes concave or “scooped-out,” an indicator of airflow limitation.
This scooped-out shape occurs because the airways have narrowed or are prone to collapse during forced exhalation. In conditions like asthma or emphysema, inflammation or loss of elastic support causes the small airways to narrow prematurely. This narrowing obstructs airflow, especially during the later phase of exhalation.
The PEF may be reduced, and the time it takes to exhale is often prolonged. The total lung volume, or the width of the loop, may be normal or even appear larger. This is due to air trapping, where airway obstruction prevents the lungs from fully emptying.
Restrictive Lung Disease Patterns
The flow volume loop for restrictive lung diseases, such as pulmonary fibrosis or sarcoidosis, presents a different pattern from obstructive conditions. The primary characteristic is its size; it looks like a miniature version of a normal loop. The shape maintains the healthy peak and expiratory slope but is significantly scaled down.
This smaller size reflects the underlying issue in restrictive lung disease: a reduction in total lung capacity. These conditions cause the lungs to become stiff or prevent the chest wall from expanding fully. Consequently, the Forced Vital Capacity (FVC), represented by the loop’s width, is reduced.
Because the problem is with volume and not airflow obstruction, expiratory flow rates can appear normal or even faster when adjusted for the small lung volume. This results in a loop that may look tall and narrow, sometimes described as a “witch’s hat,” which contrasts with the wide, scooped-out shape of an obstructive loop.